1. Field of the Invention
This invention generally relates to RF communications and more specifically to an N-way combiner that facilitates the control of a transmitted RF signal.
2. Description of Related Art
Wireless RF applications, particularly in the 800 to 1000 MHz and 1900 to 2400 MHz ranges, have become wide spread in recent years. These are frequencies of choice for wireless telephones and similar devices. Particular effort has been directed to the development of the high-power RF transmitting facilities for such applications including wireless telephone repeaters.
Many of these applications include multiple amplifiers to provide an appropriate RF output power. For example, a 600 watt transmitting facility may include four 150 watt transmitters operating in parallel, rather than a single 600 watt transmitter. Using lower powered amplifiers provides reliability through redundancy and in many cases reduces costs as the cost of several lower powered RF amplifiers may be less than the cost of a single high powered amplifier. Moreover, the use of lower powered amplifiers allows different sites to be configured at different power levels without requiring different amplifiers. For example, a single amplifier could be used to provide a 150 watt transmitting facility; two amplifiers, a 300 watt transmitting facility; etc.
However, a single, high powered transmitter is characterized by simplified impedance matching to an antenna or other RF load. Generally the impedance match remains essentially the same for a given frequency regardless of the power being transmitted. With parallel, identical, lower powered amplifiers, however, the problem becomes more difficult because the output impedance of the collective amplifiers will be Z0/N where Z0 is the characteristic impedance of one amplifier and N is the number of amplifiers operating in parallel. Thus, the impedance at a common node for a four-amplifier transmitting facility will vary between 50 ohms and 12-xc2xd ohms depending upon the number of amplifiers operating in parallel. If the impedance is not well matched, VSWR and insertion losses increase.
A number of power dividers and combiners have been proposed for minimizing the effects of impedance mismatches. Generally in these systems a single RF source produces an RF signal that divides into equi-phase, equi-amplitude input signals to parallel amplifiers. The combiner section then recombines the four amplified outputs to produce the high powered RF output signal. One particular approach, known in the art as a Wilkinson circuit, uses transmission lines at a characteristic impedance to convey signals to different ports. The ports are tied through resistors to a common node. The transmission lines may be anywhere from a quarter wavelength (xcex/4) to a half wavelength (xcex/2) in length. In such systems, however, optimal performance occurs when all parallel paths are energized. Insertion losses when only one amplifier is operating can become 75% of the input. With these losses it can be seen, particularly if equal amplitudes and phases are not maintained, that significant heat will be generated. In systems using resistors, this heat can lead to circuit failure.
U.S. Pat. No. 4,893,093 (1990) to Cronauer et al. discloses a switched, power splitter in which a high frequency input signal is applied to a plurality of amplifiers. First transmission lines connect between the input and each of the amplifiers with each transmission line capable of being switched between a high level and a low level of impedance. A balanced resistor network is preferably coupled between the first transmission lines. Second transmission lines shunt across the first transmission lines and the impedance of each second transmission line can be altered to a predetermined percentage of the circuits input impedance. A control circuit switches the various transmission lines so that the impedance of the antenna remains balanced no matter how many of the first transmission lines are in the high impedance state.
U.S. Pat. No. 5,767,755 (1998) to Kim et al. discloses another embodiment of a power combiner with a plurality of transmission lines connecting a plurality of inputs to an output terminal. RF switches provide the selection of up to N channels as active channels. The electrical length from each RF switch to the output terminal is preferably one-half wavelength at a central frequency (i.e., xcex/2 at f0). When a switch is on, the signal power applied to all of input terminals is combined at the output terminal. When the switch is off, the RF power incident to the switch is reflected and the transmission line connected between that switch and the output terminal appear as an open circuit. However, it does appear the output impedance at the combined circuit can vary over a range of 4:1.
U.S. Pat. No. 5,867,060 (1990) to Burkett, Jr. et al. discloses still another embodiment of a power combiner that will allow the selection of a number of amplifiers operating in parallel for driving a load having characteristic impedance. Each amplifier connects to a common node through a phasing line one half-wavelength at the characteristic impedance. A quarter wavelength transforming line then connects the common node to the load. This transforming line has an impedance that depends upon the number of circuits being energized simultaneously. Therefore it appears that in this system a wide range of mismatches can still occur.
U.S. Pat. No. 5,872,491 (1999) to Kim et al. disclose a Wilkinson-type power divider/combiner that has a selective switching capability. The switchable power divider/combiner includes N first switches connecting N input/output transmission lines to a common junction and N second switches connecting N isolation resistors coupled to the N input/output transmission lines to a common node. The activation of each pair of the first and second switches to a closed or opened switch position controls the operating mode. Optimal impedance matching is provided by adjusting the impedance values to provide optimal impedance matching in both N-way and (Nxe2x88x921)-way operating modes. While this system appears to optimize for a particular configuration in anticipation of a failure of one path, it does not appear readily adapted for providing optimal impedance if more than one channel becomes inactive.
Examination of each of the foregoing patents and other representative prior art indicates that each of the approaches is overly complex. Problems of heating, insertion losses and impedance mismatches continue to exist. What is needed is a power combiner that can provide good VSWR and insertion loss characteristics over a wide range of input powers.
Therefore it is an object of this invention to provide an RF power combiner that is simple to construct and cost effective.
Another object of this invention is to provide an RF power combiner that exhibits a low VSWR for a wide range of operating power.
Still another object of this invention is to provide an RF power combiner that exhibits low insertion losses for a wide range of operating power.
In accordance with one aspect of this invention, a power combiner provides a plurality of RF input connections and an RF output connection. Switched RF feed lines connect each RF input connection to a common node. An RF output conductor connects the RF output connection to an output node. An impedance matching network connects to the common and output nodes and includes capacitive stub means connected to the output node.
In accordance with another aspect of this invention, a power combiner can produce at an RF output connection the combined outputs of up to four RF inputs in response to remotely generated selection signals. A transmission line and RF input switch connect in series between each RF input connection and a common node with said RF input switch being proximate the common node. An RF transmission line connects an output node to the RF output connection. A single stub matching circuit connects to the common and output nodes thereby to establish an impedance transformation function between the common and output nodes.